1. Technical Field
The present invention pertains to a transmission system, and more particularly to a mechanical continuously variable transmission particularly suited for human-powered vehicles.
2. Description of the Related Art
Continuously variable speed transmissions are used for power and rotational motion transmission in a variety of applications where continuous variation of input to output ratio is beneficial. Variable speed transmissions attempt to provide for a seamless transition throughout the available speed range. This can be particularly challenging when the input to the transmission, such as torque and speed input, has a cyclic variation, such as that generated by a rider pedaling a bicycle crank.
The torque and speed input to the pedals of a bicycle vary at twice the frequency of the pedaling rotation with an approximately sinusoidal waveform. The relationship between average pedal RPM and bicycle speed on a standard bicycle is not constant because the torque input to the pedal shaft is approximated by a sine function of the rotation of the pedal shaft. FIG. 1 is a graph illustrating the effective force in pounds as a function of the pedal crank angle in degrees. This graph shows the effect of force (sum of both legs) at the bicycle wheel plotted against crank angle in degrees. (See, P. R. Cavanagh and D. J. Sanderson, “The Biomechanics of Cycling: Studies of the Pedaling Mechanics of Elite Pursuit Riders” as published in the Science of Cycling, pages 91-102.)
Although a bicycle rider may feel that the force applied by their legs to the pedals is steady, the graph of FIG. 1 shows that the force is neither constant nor steady. Thus, a variable-speed transmission for a bicycle would have to accommodate the cyclic speed variations and their consequent reflected torque impulses in a manner that provides a smooth transmission in the effective ratio between the input and the output and that provides an even feel to the rider.
A large body of art has been developed in this field, which will be described in more detail below. These approaches include variable speed belt drives, typical fixed-ratio, discreet-ratio, or continuous-ratio (epicyclic) gear drives, tractions drives, impulse drives, periodic drives, and the like. The traction drive in various forms is a popular variant due to some distinct benefits relative to other concepts, such as impulse drives.
Impulse drives rely upon adjustable lever arm ratios to generate a variable gear ratio. Such lever arm ratios are most often achieved by eccentric offset of a driving and driven member, with load transmitting elements in between. These load transmitting elements are numerous, and alternately carry the load for a short period when the lever arm ratio is as desired, and relax during the rest of a cycle via the use of one way clutches, ratchets, or pawls. For this reason, these drives are also referred to as periodic drives. The alternate load carrying of the load transmitting elements and the underlying kinematic motion result in speed variation during a cycle of an impulse drive.
Traction drives provide smooth operation by the use of a rolling radius ratio between a driving and driven member. The simplest concept of a traction drive is two wheels rolling together, which is a fixed gear ratio. Their operation is smooth, i.e., there are no speed variations through the unit for a fixed gear ratio setting, and there is no need for the use of one way clutches, ratchets, or pawls.
Continuous gear ratio variation in a traction drive is obtained by using driving and driven members of unique shapes in which a rolling radius can be changed on the fly. Traction drives are often characterized by the transference of torque from a rotating input member through a rotating intermediary member to a rotating output member. The intermediary member contacts the input and output members at various radii that effect a change of ratios between the input and the output members. The torque is transmitted through the system by traction or friction between the input, output, and intermediary members. A fluid can be used within the traction drive that becomes solid at some applied compression force to enhance the traction.
The use of intermediate members also provides the opportunity to further change gear ratio and increase the gear range. Some means is provided for varying the rolling radius ratio by mechanical adjustment of one or more parts. Shifting of a traction drive is relatively easy because the shifting path is generally perpendicular to the primary load transmission path. The patents of Blake (U.S. Pat. No. 5,597,056), Lutz (U.S. Pat. No. 5,318,486), and Schievelbusch (U.S. Pat. No. 5,273,501) are exemplars of such systems for bicycle applications using cone-, toroid-, and ball-type traction devices respectively.
The device disclosed in the patent of Kitchen and Storey (U.S. Pat. No. 1,083,328) is another example of an industrial traction drive that is particularly compact due to the use of a hemispherical geometry. This patent also shows a variety of arrangements of intermediate members to achieve different effects, such as reverse gears.
Such traction drives are well proven and in extensive industrial use. They are generally assumed to be most useful at high speeds and low torques so as not to overload the traction mechanism, which can result in slipping. Efficiency, however, decreases with increased torque due to micro-slip. Also, traction drives are not used in situations where a shifting between gear ratios occurs often.
The primary disadvantage of continuously variable traction drives is that they must rely upon friction for their operation. Gear teeth cannot be used because of the continuously variable geometry during shifting of the gear ratio.
Traction drives in industrial use commonly use metallic parts, and friction is developed through high normal pressure between the parts, often in conjunction with special traction fluids which enhance the friction. The high normal pressure required to develop useful traction results in heavy parts to take the loads. In some cases, input gearing is used to increase internal speeds and reduce required traction, but at the expense of efficiency losses in the additional gear meshes. The use of metals, which are very stiff, also requires the parts to be made with very high precision, like a rolling element bearing, leading to high cost of manufacture. Finally, the very stiff material used and the variable geometry of rolling introduces relative sliding between parts and resulting wear. Indeed, the inventions of the exemplar patents mentioned above are not currently on the market, probably for these reasons.
Another CVT drive type that operates by traction is a rubber belt drive with variable radius sheaves, such as a typical snowmobile transmission. These drives are not compact or lightweight to the extent that the present invention enables, as will be described below. Also, those systems do have force feedback to the shifter, which in the case of a snowmobile is automatic, but they can be a nuisance in other applications.
Attempts have been made to apply traction drives to bicycles. However, conventional traction drives suffer from weight problems due to the need to react high normal loads that are necessary to generate traction. One proposed traction drive, known as the toroidal traction drive, utilizes a driving member and driven member that are toroidal shaped, and an intermediate wheel that transmits the load. Some designs require a third member to react the normal forces. Examples of these types of drives can be found in U.S. Pat. Nos. 4,735,430, 4,858,484, 4,964,312, 4,086,820, 5,020,384, and 4,934,206. While these drives offer advantages such as low “stutter” (such as vibrations caused from torque feedback due to the cyclic variations at a different frequency from the input rotation), high efficiency, a compact space, large gear ranges, and may be automated for torque response, they also have the drawbacks of high contact loads requiring heavy parts, inability to shift at zero speed, and low shifter force and feedback in shifting the toroidal traction drive.
In a ball traction drive, such as that disclosed in U.S. Pat. No. 5,318,486 for a driving hub for a vehicle, the driving and driven members are shaped like typical bearing races, and a ball (35) transmits the load. Such designs require a third member to react the normal forces, as also disclosed in U.S. Pat. No. 5,236,403. While these designs offer little stutter, are compact and efficient, and may be automated and have large gear ranges, they generally have high contact loads that require heavy parts, cannot shift at zero speed, and have low shifter force and feedback.
Another design involves a wheel on a disc traction drive, such as disclosed in U.S. Pat. Nos. 4,819,494 and 5,626,354, where a wheel comprises the driving member, and the driven member is formed from a plate or disc. These designs suffer from not having a robust third member to react the normal forces. Some designs use a pair of pre-loaded discs to increase reaction capability, which helps in eliminating stutter and facilitates automated or torque response. However, these designs have high contact stresses compared to other designs and cannot shift at zero speed.
Other designs include the ring-and-cone traction drive and the ball-on-disc traction drive. The former is widely used for industrial purposes and uses an inclined cone against a cylindrical ring with traction load transmission, the ring moving axially along the cone to change the gear ratio, resulting in low stutter and high efficiency. However, this design is too large for a bicycle and has complex shifting mechanisms, limited ratios of gearing, and an inability to shift at zero speed. The latter utilizes discs to transmit a load by multiple balls, which provides load sharing and lower contact stresses, thus achieving simplicity, low part count, and a more simple shifter, but requiring offset shafts that are large, having an inability to shift at zero speeds, and a low efficiency due to ball carrier friction.
Yet another design is that found in U.S. Pat. No. 1,083,328, referenced above, for a variable-speed friction gearing wherein first and second hemispherical halves comprise driving and driven members that are interconnected by at least one pair of idler wheels that vary their contact position along the inside of the spheres. While this design shows promise for applications to bicycles, it also suffers from high contact loads, and an inability to shift at zero speed. Its advantages, however, include low stutter, compactness, low parts count that are inexpensive to manufacture, high efficiency, and a gear range of 6 to 1 in small size drives with low shifter force and torque.
Another type of transmission is where there is a periodic connection between the input and output rotating members to transfer the torque from the input to the output. This type is characterized by a cyclic variation in the output rotation speed with a constant input rotation speed. The cyclic variations in the output rotation speed reflect a torque variation to the input when there is a substantially constant torque load. Many methods have been proposed to mitigate the variation, typically involving complex mechanisms that do not eliminate the variation completely.
In U.S. Pat. No. 4,873,893, an infinitely variable positive mechanical transmission is disclosed that, in a first embodiment, transmits power from an input shaft to an output shaft utilizing a gear on the output shaft meshed with an idler gear that is coupled to the input shaft through a wobble plate mounted on the input shaft and a connecting rod coupled between the wobble plate and the idler gear. This particular approach requires the use of multiple gears, in this case a plurality of idler gears that are meshed with the gear on the output shaft and each of the idler gears is coupled to the wobble plate through a connecting rod and a one-way gear or brake.
Another variation utilizes an eccentric member to move an element away from a main rotary input axis of the device and a clutch to periodically connect the rotary input motion to the rotary output motion. An example of this approach would be the use of a ratchet and pawl as a speed variator, done by handing off the connection from one pawl to the next as the relative speed of one pawl to the ratchet overtakes that of another pawl. An eccentric element is used to cause the hand-off to occur at a point of maximum relative velocity between the ratchet and the pawl. The disadvantage of this attempt is that the instantaneous matching of speed at the maximum relative velocity between the parts creates a very large force in addition to the useful torque being transferred between the ratchet and the pawl. This can result in unevenness in the output, and it increases wear on the parts, requiring heavier and more costly drive train components.